But have they been complaining about it? Iron Man 2 wasn't unsuccessful at the box office.

Jeez come on man, it was always going to be successful coz of film one, but that wasn't an excuse to force things in. And yes I do know people who did complain, they had no idea what the hell what some of the stuff was referring to, given how some of the characters were introduced it's easy to see why. Basic rule of film making - you don't pander to the fan base and assume everyone else knows what the hell you're talking about. Latest Harry Potter suffers from the exact same thing, without prior knowledge of both films and worse the books you could easily get lost in some parts. People make out like this is one big series of sequels, it's anything but, this is several different story lines that somehow have to be made to meet in the middle, but they've gotta work individually first and foremost, if they require knowledge of other movies or knowledge of what's coming up ahead or worse, the comic mythologies, in order to be understood then it's poor film making. Prior knowledge of a character or universe or book should never be a requirement before hand, the films need to work in their own right. If characters and events need to be there, then it's up to the writers to ensure they work with the story and not have the story moved to accommodate them. The problem in what Marvel are doing is that they're trying to make a concept work in a medium that it isn't suited for. It works for TV, hell that's more comparable a medium to comics than what films will ever be. Somewhere along the way things are gonna start getting compromised in trying to link things that aren't relevant to the story at hand. IM2 got a free pass in many respects due to film one's success, what if Thor or Cap have equally tacked on plot devices that distract and bog down the film? Given they don't have the luxury of a successful film one it's hard to see them being given the same leniency.